# Durability / fatigue resistance

**Load this doc when:** the question is about fatigue resistance, why late-race power fades,
training the ability to hold power deep into long efforts, or running the durability field test.

**Data/knowledge split:** the MCP server gives you the kJ, power curves, and fatigued-vs-fresh
efforts. This doc explains *why fatigued-state power matters* and *how to train it*.

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## Durability is a distinct, trainable quality

"Durability" is the time of onset and the magnitude of deterioration in physiological-profiling
variables (thresholds, efficiency, power) over the course of prolonged exercise — a quality that
should be profiled **separately** from fresh-state numbers, because two riders with identical fresh
profiles can fall apart very differently after several hours (Maunder et al. 2021, *Sports Med*
51(8):1619–1628; PMID 33886100; DOI 10.1007/s40279-021-01459-0).

### Why it matters more than fresh power

- In professional cyclists across a Grand Tour, **fresh** mean-maximal power did **not** separate
  WorldTour from ProTeam riders — but as work accumulated (0 → 35 kJ·kg⁻¹), WorldTour riders held
  higher power. **Fatigued-state power differentiated performance; fresh power did not**
  (Muriel et al. 2022, *Eur J Sport Sci* 22(12):1797–1804; PMID 34586952;
  DOI 10.1080/17461391.2021.1987528).
- The **fatigued** power profile varies far more across a season than the fresh profile, and it
  tracks with **training characteristics (accumulated volume/load)** — meaning durability is
  something you build with training, not a fixed trait (Spragg, Leo & Swart 2023, *Eur J Sport Sci*
  23(4):489–498; PMID 35239466; DOI 10.1080/17461391.2022.2049886).

## The load dependency (the key training implication)

**Durability depends on maintaining high overall training load, whereas fresh power can be held on
reduced load.** You can taper volume and keep your 5-min power; you cannot taper volume for long
and keep your *fatigued* 5-min power. This is the empirical thrust of the Spragg training-
characteristics paper (PMID 35239466); the crisp "reduced load keeps fresh power, sustained load
keeps durability" phrasing is Spragg's applied interpretation of that data — cite it as such.

Physiological correlates of *who* is durable (higher VO2max, better gross efficiency, higher fat-
oxidation) come from the companion paper (Spragg, Leo & Swart 2023, *Med Sci Sports Exerc*
55(1):133–140; PMID 35977108; DOI 10.1249/MSS.0000000000003024) — useful for understanding
mechanism, but the *trainable lever* is sustained load + the practices below.

## How to train durability

1. **Long-term consistency and high volume.** Durability is a load-dependent adaptation — it is
   built over blocks and lost when load drops. Protect the long ride.
2. **Prolonged sessions with hard efforts placed late (fatigued-state intervals).** Put the quality
   work *after* accumulated kilojoules, not at the fresh start of the ride — you are specifically
   training the fatigued state that discriminates performance.
3. **Glycogen-sparing nutrition.** Fuel long sessions to spare glycogen; the goal is training the
   durable state under realistic fueling, not chronic depletion. (Fatigued power is glycogen-
   sensitive — see the fueling caveat in `data-confounds.md` for why HR is unreliable here.)

> ⚠️ **Under-researched — track individual response.** The *mechanism* (load-dependent fatigue
> resistance) is well supported, but the **optimal durability dose** — how much accumulated work,
> how often, how late to place efforts — is not established. **Do not assume a fixed protocol.**
> Track each athlete's fatigued-state response and titrate.

## Measure it: the fatigued-state field test

Use `../assets/field-test-durability.md`. In brief:
- Establish a **fresh** 1-min max on a fixed hill.
- On a separate day, accumulate **~1,500 kJ (~2.5 h)** of mostly-endurance riding, then repeat the
  **same 1-min max on the same hill**.
- `Durability % = fatigued ÷ fresh × 100`. Track the trend across a block.
- Hold the hill, gearing, kJ pre-load, and fueling **constant** so the comparison is valid.
- **Judge from power, not HR** — HR after 1,500 kJ is confounded by drift, heat, and fueling
  (`data-confounds.md`).

## Discipline note

Durability matters for both road/TT and MTB, but **keep absolute numbers discipline-specific.** The
1,500 kJ anchor is a sensible generic default, not a validated cross-discipline constant; a masters
marathon-MTB rider and an elite road pro do not share a durability dose. See `mtb-xco-demands.md`
for how fatigue resistance shows up specifically in off-road racing.